Project Summary Opioids such as morphine are effective at relieving pain but can also be addictive due to their rewarding properties. Brain areas that include the medial and anterior thalamus, prefrontal and anterior cingulate cortices, and the dorsomedial striatum and nucleus accumbens are involved in the affective and motivational aspects of pain perception. Projections from cortical and thalamic regions converge on the striatum providing two important sources of excitatory innervation to the limbic system and basal ganglia. This proposal will use viral based gene delivery to express light-gated ion channels in thalamic and cortical brain regions to achieve selective excitation of thalamo-striatal and cortico-striatal glutamate afferents in the dorsomedial striatum of mice. Whole cell recordings will be used in brain slices with the goal of understanding the location and mechanism of opioid receptor action within this medial pain pathway. The first aim will address the location of ?-opioid receptors (MOPr) in these pathways. The hypothesis is that the major effect of opioids is to inhibit thalamic projections to the striatum and cortex through a presynaptic mechanism while the cortico-striatal projections are insensitive to opioids. Aim 2 will investigate the effect of chronic treatment with the clinically relevant opioids morphine and fentanyl. These agonists differ substantially in efficacy and the induction of MOPr phosphorylation. Recent work shows definitively that phosphorylation of MOPr plays a role in trafficking and function measured postsynaptically but it is not known how or if phosphorylation affects presynaptic function. Wild-type mice and a newly generated knock-in mouse expressing phosphorylation-deficient MOPr will be acutely and chronically treated with morphine and fentanyl with the hypothesis that drug-induced MOPr phosphorylation will lead to a decrease in the efficiency of opioid receptor dependent inhibition of thalamic glutamate release (receptor tolerance) following chronic opioid treatment. The phosphorylation- deficient MOPr mouse is therefore expected to show less receptor tolerance than wild type mice. The results from these aims will describe the location and action of MOPr within these thalamo-cortico-striatal circuits and determine the receptor- and cellular-level adaptations that result from chronic opioid treatment. A better understanding of this circuitry may lead to approaches or treatment regimens that better manage the treatment of pain and limit reinforcing and rewarding properties of opioids.